The volumes of exhaust gas from such sources as paper mills and power plants require not only ducts of large cross sectional areas to convey the gases to stacks but also equipment in a section thereof by which the gases are conditioned or treated before being discharged.
As the equipment within such a section may need servicing, repair or replacement or additional equipment may have to be incorporated in that section, it is necessary to enable workmen to enter that section without exposure to the gases and at the same time, make it unnecessary to shut down the operation at the plant which produces the gases. Each such duct system is accordingly provided with a duct section bypassing the section in which the equipment is located and may itself be provided with duplicate equipment. Such duct systems include means to isolate either section when not in use, from the other section.
While guillotine dampers are used for isolating each section, valves of the diverter and isolator types have certain advantages over guillotine dampers as such a damper requires that the blade in the open position, protrudes outside the duct and then be stored in a bonnet or provided with additional seals at the throat where the blade of the damper exists from the duct. Valves which can be moved between positions by toggle joints are also advantageous because the valves may be quickly shifted from one position to another.
A diverter valve, see U.S. Pat. No. 4,821,507, includes a body incorporated between an upstream section and the downstream section containing the equipment. The body has aligned inlet ports and an additional port disposed at a right angle to the flow path between the aligned ports. A blade within the diverter body is connected thereto to swing between two operative positions, in one of which flow through the aligned ports is permitted and the additional port is closed and in the other operative position flow is only permitted through the other aligned port and the additional port. Such diverter valves and the seats surrounding the outlet port or ports are marginally sealed by means which, in the above referred to patent are flexibly resilient leaf springs which are shown as in accordance with U.S. Pat. No. 4,823,836 and which are disposed to seal the margins of the blade whenever it is operatively positioned.
An isolating valve includes a body which is essentially a frame dimensioned to be incorporated between duct sections and establish a flow path between the sections and bordered by a seat. A blade is pivotally connected to one side of the frame to swing between an operative position blocking the flow path and an inoperative position at one side thereof. In the operative blade position, leakage about the margins of the blade is prevented by sealing means.
The effective sealing of the blades of either diverter valves or isolating valves presents problems since, with either type, the ducts are of a large cross sectional area with a duct twenty by thirty feet being typical. The ports and valve seats are accordingly so large that the surfaces of the seat are likely to have areas which in fact are slightly uneven and constitute potential leakage paths thus making the use of sealing air a necessity.
In the above referred to U.S. Pat. No. 4,821,507, the seats of a diverter valve are shown as provided with surrounding, spaced apart seals which are engaged by appropriately spaced and dimensioned seats surrounding the blade, to provide, when the blade is operatively seated, a passage about the blade for sealing air by which exhaust gas leakage is prevented with a volume of sealing air such that air will be forced outwardly through any such leakage path.
Leaf spring seals, as presently employed, are initially effective particularly when an adequate volume of sealing air is also used. Such seals, when used prior to the present invention, become less effective in use with a leading cause that the seals and seats are brought, in use, in face to face contact with sealing pressures applied directly against them. Additional causes of ineffective sealing are associated with either blade or seat encrustments or both developing during use.